1. Field of the Invention
The invention relates to a process for preparing 1-cyclopropylalkane-1,3-diones from an alkali metal alcoholate, a carboxylic ester having at least one hydrogen atom on the alpha carbon of the carboxylic acid skeleton, and a cyclopropyl alkyl ketone.
2. Discussion of the Background
Cyclopropylalkane-1,3-diones are precursors of numerous pesticides. It is known in the literature to prepare cyclopropylalkane-1,3-diones from cyclopropyl alkyl ketones and carboxylic acid derivatives, with the carboxylic acid species which are most reactive, such as for example, acetic anhydride or acetyl chloride, being reacted with, for example cyclopropyl methyl ketone (CPMK), and the reaction being carried out in the presence of aggressive catalysts or auxiliary substances, e.g. BF.sub.3 (U.S. Pat. No. 3,507,958).
The use of acetyl chloride or BF.sub.3 in industrial processes is associated with a particularly high degree of technical complexity owing to the highly toxic and strongly corrosive properties of these hydrolysis sensitive chemicals and their decomposition products.
Carboxylic esters, such as for example ethyl acetate, represent suitable synthesis partners for the cyclopropyl alkyl ketone, for example CPMK, which can be handled more easily and the use of which is associated with a significantly lower hazard potential. However, the corresponding condensations were hitherto achieved using the strongest possible bases, such as, for example, sodium hydride (U.S. Pat. No. 3,507,958) or sodium amide (Cannon and Widden, J. Org. Chem. 17,685 (1952)), with the yields being relatively unsatisfactory, i.e. from about 40 to 75% of theory.
The use of sodium amide and sodium hydride naturally involves substantial risks, since explosive products form in the presence of moisture, air and carbon dioxide, and, therefore, necessitates extensive safety precautions.
In addition to this, the syntheses carried out using sodium hydride are typically associated with uneconomical space/time yields, with reaction times of up to 14 hours.
It appears from EP-A-0 410 726 that there is no benefit in dispensing with such strong bases and replacing them, for example, with less basic alkali metal alcoholates of lower alcohols, such as, for example, sodium methoxide or sodium ethoxide. According to this reference, reaction of CPMK and ethyl acetate together with sodium methoxide only achieves a yield of about 21% of theory and a grade of 75%, even when the condensation is carried out at high temperatures. The alcohol is continuously distilled off from the reaction mixture in order to displace the equilibrium, and in addition to this, the ethyl acetate is employed in a large excess. In order to separate off the byproducts, a filtration step is required which is elaborate for an industrial manufacturing process and in which effective precipitation is not achieved at a purity of 75%, thereby necessitating additional work-up steps before the product can be marketed. The unsatisfactory yields not only represent an economic problem, but must also be criticized from the ecological point of view since it is necessary to dispose of relatively large quantities of byproducts.